Optomechanics – Samuel Colt’s Principle of Interchangeable Parts


Well, San Diego’s history now.  Whew!

Thanks to the OMTG Program Committee for an absolutely terrific two-days of papers (plus a poster session).  Thanks to Phil Pressel for an awesome evening presentation on the Hexagon camera system.  And thanks to Eugene Arthurs and the SPIE staff for keeping it (all the rest of the Symposium) together:  What a herd of cats!

And if you missed the Exhibit Hall, well that’s your problem, understandable but still your problem.

Back to optomechanical engineering.  One of the mechanical engineer’s duties on an optical project is to survey the available mechanical design space looking for problems.  The mechanical design space includes dimensions, temperatures, stresses, deflections, tolerances, pressures, masses, damping, friction, durability, service life, stability, ….  Oh, I shouldn’t leave cost out of the design space either. 

In my practice of the mechanical engineering arts I’ve become a disciple of Samuel Colt.  He’s the guy who introduced the principle of interchangeable parts to the manufacture of his infamous .44 caliber revolver in 1841.  Up to that time firearms were assembled by a gunsmith who would grind, file and polish all the manufactured parts until they fit together and operated to his satisfaction.  Their weapons were very expensive.  On the other hand the Colt revolver’s price was so low that “The Great Equalizer” became available to almost everyone.

My tolerancing method applies Colt’s principle to optical products.  Using influence coefficients from my AEH/Ivory Optomechanical Modeling Tools, I calculate the maximum worst-case assembly errors between the image and the detector in all seven registration variables:  Tx, Ty, Tz, Rx, Ry, Rz and dM/M.  I include the tolerances on the lens design variables (R1, R2, t and n) in addition to all the mechanical dimensional tolerances.  Then I tweak all the tolerances (in a spreadsheet) so that the pain is equally shared between the mechanical suppliers, the optical suppliers and the assembly technicians.  And, all the manufactured parts get used as-is. 

When I describe this principle someone is usually perplexed at how I can do this without using the statistical distribution of each dimension.  I point out that I can put the statistical distributions into the calculations if I choose but they won’t change the maximum worst-case assembly errors.

Scrap is another one of the problems that mechanical engineers work to avoid.  Thank you Samuel!

Well, I bought some candy corn this morning.  All Hollow’s Eve is on the way.


Al H.

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